Almost all electronic circuits require a source of direct current (DC) power. Batteries, of course, may be used for low power systems, but generally electronic systems are powered by a power supply which converts alternating current (AC) from commercial power lines to DC power.
Electronic systems, particularly monolithic integrated circuits, are designed to operate over a relatively narrow voltage range. The recent advent of large scale integrated (LSI) and very-large scale integrated (VLSI) technology produces devices particularly susceptible to denegration or less than satisfactory performance when the input voltage varies other than to a slight degree. Unregulated power supplies provide an output voltage which varies with the load, the AC input and temperature. Those variations are devastating to sensitive electronic systems. Accordingly, sensitive electronic systems necessitate the use of a regulated power supply; that is a power supply which is capable of providing a DC voltage output which is independant of the current drawn by the load, of variations in input line voltage, and of temperature.
Various regulated power supplies for electronic circuits have been designed and are in use. Indeed, fixed-voltage regulators of monolithic design, for example the Motorola MC 7800 C series, are currently available. Such regulators provide an industry standard voltage output of, for example, five (5), twelve (12), eighteen (18) or twenty-four (24) volts. The advent of such monolithic regulators has allowed system design whereby unregulated voltage is provided throughout electronic systems and regulation is provided on individual printed circuit boards. Thus, substantial flexibility in design, regulation, isolation and decoupling is possible.
Such regulators (sometimes referred to as series pass regulators), however, suffer from several disadvantages. If the electronic system requires a plurality of differing voltages and there is only one DC supply voltage, system design may prove the addition of multiple unregulated DC voltages uneconomical or impractical. Additionally, in systems where all voltages must be generated from a single DC voltage source, and the system requires multiple different DC voltages, DC-to-DC converters are necessary. Another disadvantage is that such regulators require the input voltage to be greater than the output voltage and are generally inefficient.
The use of a switching regulator obviates those disadvantages. The basic switching regulator consists first of a reference regulator which may be a series pass regulator as described above. The output of that reference regulator is a fixed reference voltage which serves as the power supply voltage for the other circuits in the switching regulator. A difference or error amplifier compares the fixed reference voltage with a fraction of the output voltage of the switching regulator. The output voltage from the difference or error amplifier is applied to the inverting terminal of a pulse width modulator. A triangular wave form is applied to the noninverting terminal of the pulse width modulator. The modulator, in known fashion, thus produces a square wave having a duty cycle which varies linearly with the output voltage of the difference or error amplifier. The output of the pulse width modulator, in known fashion, is connected to drive a power switch thereby creating a square wave having a minimum value of zero and a maximum value equal to the unregulated input voltage to the switching regulator. The output voltage of the switching regulator will therefore be constant and will be regulated by varying the duty cycle of the square wave.
Many switching regulators are, of course, available on the market in single package design. The Silicon General 3524 is such a switching regulator. That switching regulator is useful in power supplies for electronic systems, particularly computer terminals and other microprocessor systems, for converting unregulated DC input to plurality of varying voltage DC outputs. By adjusting the duty cycle as aforesaid, the regulator converts an unregulated input of, for example, twenty (20) to forty (40) volts to, for example, a regulated constant a twelve (12) volt output. In other words, the output voltage and current are dependent upon the duty cycle of the switching regulator.